PROJECT SUMMARY/ABSTRACT Drug overdose is now the most common cause of injury death in the United States, and opioid analgesics are involved in almost half of these deaths. Overdose deaths have quadrupled since the late 1990s, corresponding to rising misuse of opioid analgesics and heroin. Increasing access to medication-assisted treatment (MAT) such as methadone maintenance and buprenorphine is one important strategy for addressing opioid use disorder (OUD). Expanded Medicaid eligibility for low-income adults could increase use of OUD treatment among low-income adults with inconsistent insurance coverage. Expanding eligibility for Medicaid may be most effective in reducing OUD if Medicaid programs offer comprehensive MAT coverage and treatment providers locate in areas where new Medicaid enrollees are likely to require services. This application for a National Institute on Drug Abuse Mentored Research Scientist Development Award (K01) seeks support for a new Assistant Professor at the Johns Hopkins Bloomberg School of Public Health who aspires to a career focused on improving publicly-funded services and systems that treat individuals with substance use disorders, particularly OUD. Dr. Saloner's prior research has focused on access to care for behavioral health disorders and on health insurance. To extend the reach and scope of this research, Dr. Saloner proposes to move his research toward clinical outcomes, resource allocation and value measurement, impact evaluation, and geospatial analysis. Accordingly, the K01 training is focused on gaining knowledge in addiction medicine, cost effectiveness analysis, causal inference, and spatial statistics. The proposed research will employ mixed methods to study the multiple determinants of access to OUD treatment for low-income adults. Research aims will include (1) an evaluation of how expansions of Medicaid eligibility for low-income adults have impacted use of treatment services, health, and social outcomes among low-income adults with OUD; (2) a geographic analysis to identify clusters where individuals with OUD reside, and to examine the proximity of those clusters to treatment resources and to areas with high Medicaid enrollment; (3) a detailed analysis of how access to medication-assisted treatments in two different state Medicaid programs is associated with treatment costs and health outcomes. The practical application of this work will be to guide efforts by states and localities to more effectively target resources to individuals with OUD. By providing salary support, training, freedom from administrative responsibilities, and a formal mentorship structure, this award will facilitate Dr. Saloner's transition to an independent research career focused on improving health and social outcomes among persons with OUD.